In a CMOS image sensor, a pixel detects an image by accepting light rays in an area on which a photodiode is formed. A photodiode is a device that produces electrical signals by generating electron hole pairs (hereinafter referred to as “EHPs”) by means of incident light rays through a p-n junction.
At present, a commercialized process technology uses 0.35 micron (μm)˜0.50 μm technology, where a pixel size is 7˜8 μm. In case of two-dimensional pixel architecture, in a process technology of 0.25 μm, the physical limit of pixel size is about 4 μm×4 μm.
However, there is a technical problem in case of systems on chip (hereinafter referred to as “SoC”). In particular, SoC use a process technology of less than 0.25 μm to integrate many constituents on a single chip. Accordingly, the pixel size of a CMOS sensor must be reduced and the number of EHPs produced must remain at the existing level so that the sensor can separate image signals from noise and parasitic components.
A shallow trench isolation (hereinafter referred to as “STI”) is one technique that can be used to isolate devices such as memory cells or pixels from one another. However, STI methods may cause a dark current due to defect centers by etch damages around edges. To eliminate this disadvantage, the photodiode region is generally doped deeply by an ion-implanting process so as to cover the STI. However, in that case there is a problem in that signals with short wavelengths near blue region in visible rays get buried.
FIGS. 1a to 1c are cross sectional views illustrating a process of forming a pixel for a CMOS image sensor according to the prior art. Referring to FIG. 1a, a CMOS image sensor is formed on an epitaxial wafer having a structure of an epitaxial layer 2 doped with a low concentration positioned on a p-type substrate 1 doped with a high concentration. Active areas are isolated from one another by STI layers 3.
Next, referring to FIG. 1b, after patterning a photoresist layer 4 on the active area formed, an ion implantation process is conducted in order to form a photodiode p-n junction. There is a profile 6 doped in the epitaxial layer 2 doped with a low concentration.
Then, referring to FIG. 1c, the photoresist layer 4 is removed, and an annealing process is performed. A final profile 7 doped in the epitaxial layer 2 forms a junction under the STI layer 3 to reduce a recombination current generated at the STI edge.
As mentioned above, the existing method for making a pixel for a CMOS image sensor has to make a deep photodiode junction because of a recombination current problem due to defects and damages of STI edges. However, in that case, the signals of blue wavelength appear near the surface. In addition, in a two-dimensional structure the active area and pixel size are the same, and, therefore, a degree of integration is reduced.